[JIT] Improve error message for unsupported Optional types (#44054)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44054
**Summary**
This commit improves the error message that is printed when an
`Optional` type annotation with an unsupported contained type is
encountered. At present, the `Optional` is printed as-is, and
`Optional[T]` is syntatic sugar for `Union[T, None]`, so that is what
shows up in the error message and can be confusing. This commit modifies
the error message so that it prints `T` instead of `Union[T, None]`.
**Test Plan**
Continuous integration.
Example of old message:
```
AssertionError: Unsupported annotation typing.Union[typing.List, NoneType] could not be resolved.
```
Example of new message:
```
AssertionError: Unsupported annotation typing.Union[typing.List, NoneType] could not be resolved because typing.List could not be resolved.
```
**Fixes**
This commit fixes #42859.
Test Plan: Imported from OSS
Reviewed By: gmagogsfm
Differential Revision: D23490365
Pulled By: SplitInfinity
fbshipit-source-id: 2aa9233718e78cf1ba3501ae11f5c6f0089e29cd
